For anyone who has not noticed it ... you are now able to change the system's Ethernet card MAC adress without any third party software. Just do a:
% sudo ifconfig en0 ether aa:bb:cc:dd:ee:ffwhere en0 is the network interface (numbered from en0, en1, en2 ...) and aa:bb:cc:dd:ee:ff is the desired MAC address in hex notation.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031102075234315